Notesium docs: a first complete pass

[alonswartz]

I've published a first complete pass of the Notesium documentation at https://www.notesium.com/docs/

This includes a new Getting started section, expanded and updated documentation across the board, more examples, and a significantly fleshed-out Web Interface section. The site also includes a built-in search, which makes navigating the docs much easier.

The documentation currently covers:

• Getting started
• Core concepts
• Command line interface
• Web interface
• Vim plugin
• Custom URI protocol
• API specification
• Development

The docs largely overlap with README.md and API.md in the repository. I'm still undecided how much documentation should live in-repo versus on the website, so for now both exist.

Feedback on clarity, structure, gaps, or redundancies is very welcome.